Washed rabbit basophils, sensitized with antigen-specific IgE, secrete platelet activating factor (PAF) when exposed in vitro to the antigen. Rabbit PAF is a low molecular weight lipid molecule that aggregates platelets and causes them to secrete serotonin and other platelet constituents. PAF also has procoagulant activity in the rabbit. These qualities have suggested that PAF is an important mediator during allergic reactions. Previous failures to isolate PAF in vitro after exposing sensitized basophils in whole blood to specific antigen or in vivo during antigen induced anaphalaxis were due to the presence of a factor in serum that inactivates the biologic activity of PAF. This inactivation occurs within seconds after mixing PAF with plasma or serum in vitro or after the release of PAF in vivo. The factor is labile to heating to 65 degrees centigrade for 30 minutes and more importantly, its activity is destroyed by acidifying whole blood, plasma or serum to pH 3.0 for five minutes. After the presence of this acid labile factor (ALF) in plasma was appreciated, it was possible to isolate PAF from rapidly acidified blood of sensitized rabbits 60-180 seconds after the I.V. injection of antigen. A substance very similar to rabbit PAF has been isolated by others from washed human cells and during the past year we have found and partially purified an acid labile factor in human serum that inactivates human and rabbit PAF. Human ALF is in the void volume of a Sephadex G-200 column and is adsorbed from serum or partially purified fractions by anti-beta-lipoprotein but not by anti-IgM. This application proposes to refine methods to quantify levels of biologic activity of ALF in human sera and to purify and characterize ALF; to look for the generation of human PAF in vitro after adding specific antigen to whole blood from allergic persons; to look for human PAF in vivo during experimentally induced asthma after inhalation challenge with antigen; to look for low levels of ALF in serum of patients with lung diseases associated with immune disorders and thrombocytopenias; and to attempt to develop a radioimmunoassay for human ALF.